Protection and feeding of young animals is essential for survival. Parental care typically falls to the mother and females are often spontaneously maternal. In contrast, males show varying levels of parental interactions ranging from attack or neglect to full parenting of offspring. In mice, paternal care requires facilitation: virgin males typically attack pups, becoming paternal only after mating. Males display parental behavior around 12-18 days after mating. This switch in the social behavior of male mice toward pups provides a unique paradigm to study cellular mechanisms underlying opposing social responses to pups. The brain areas involved in these social responses toward pups are poorly studied, though our lab has recently uncovered the critical role of galanin-expressing neurons of the medial preoptic area in the positive regulation of parental behavior. My project aims to define neural populations involved in the negative regulation of parental behavior toward pups and to understand physiological and environmental factors influencing this behavior. To accomplish this goal, I will first determine the identity of neurons located in the perifornical area that are activated during pup-directed agonistic behavior (Aim 1). I will isolate populations of active neurons by combining in situ hybridization against immediate early gene c-fos with laser capture microscopy techniques (Aim 1a). This experiment will provide potential candidate markers specific to behaviorally relevant neuron populations. Preliminary studies indicate that urocortin 3 (ucn3) is a promising candidate with specific expression in the perifornical area and a documented role in social and stress-related behaviors. I will study the sex- and experience-dependent modulation of perifornical ucn3 cell number and gene expression level in virgin and mated males and females (Aim 1b). Furthermore, I will uncover the behavioral specificity of perifornical ucn3 cells by comparing their activity in males and females during a variety of social behaviors (Aim 1c). To functionally test the role of perifornical ucn3 neurons, I will perform loss and gain-of-function experiments (Aim 2). I have resuscitated a mouse line expressing Cre recombinase under a ucn3 promoter in the lab to facilitate use of conditional viral techniques for neuron-specific manipulation. I will specifically ablate neurons using a Cre-dependent capase 3 virus (Aim 2a) or activate them using a Cre-dependent excitatory designer receptor exclusively activated by designer drug approach (Aim 2b) to determine their role in pup- directed behavior. Lastly, I will uncover the role of perifornical ucn3 cells as a mediator of stress-induced disruptions in parental behavior (Aim 3). First, I will correlate perifornical ucn3 cell activationwith stress- induced parental behavior deficits (Aim 3a) and then attempt to abrogate these behavioral effects of stress by inhibiting ucn3 neurons using a Cre-dependent virus expressing a mutated glycine receptor activated by a pharmacologically selective actuator molecule (Aim 3b). The overall goal of these experiments is to functionally assess the role of perifornical ucn3 neurons in the negative regulation of parental behavior. In addition, I aim to understand the role of these neurons as a mediator of stress-induced parental behavior modulation. These studies will illuminate brain circuits involved in essential social behaviors and provide new entry-points to inform the diagnosis and treatment of mental disorders associated with stress-induced mood alterations.